The present invention relates to a laser-engravable recording material for producing relief printing plates, in particular for producing flexographic printing plates, comprising a dimensionally stable support and a recording layer comprising silicone rubbers and inorganic ferrous solids and/or carbon black as absorbers for laser radiation. It further relates to a process for producing relief printing plates by laser engraving of such recording materials, and to relief printing plates having a printing relief comprising silicone rubbers and inorganic ferrous solids and/or carbon black.
Increasingly, the conventional technique for producing photopolymeric relief printing plates, flexographic printing plates or gravure printing plates by placing a photographic mask onto a photopolymeric recording element, exposing the element to actinic light through this mask, and washing off the unpolymerized areas of the exposed element with a developer fluid is being replaced by techniques employing lasers. In this context a distinction should be made between essentially two different techniques:
First, it is known to provide photopolymeric relief printing plates with laser-writable layers. These layers consist, for example, of a binder containing dispersed carbon black. By irradiation with a IR laser it is possible to ablate this layer and mark an image into the layer. The image information is transferred directly from the layout computer system to the laser apparatus. From the laser-ablatable layer, therefore, a mask is produced which adheres directly to the photopolymeric printing plate. There is no longer a need for a photographic negative. Subsequently, the printing plate is exposed and developed conventionally, in the course of which the residues of the laser-writable layer are removed as well.
Secondly, in the case of direct laser engraving, depressions are engraved directly into an appropriate plate using a sufficiently powerful laser, in particular an IR laser, to form a relief suitable for printing. Subsequent photopolymerization and development of the plate are not necessary.
A key difference between the techniques depicted lies in the amount of material that must be removed. Whereas the abovementioned laser-writable layers are usually just a few xcexcm thick, so that only small amounts of the materials of which the IR ablative layer is composed must be removed, it is necessary in the case of direct laser engraving to remove large amounts of the material of which the printing relief is composed. A typical flexographic printing plate, for example, is between 0.5 and 7 mm thick and the nonprinting depressions in the plate are between 300 xcexcm and 3 mm deep.
An essential factor for the quality of the printing relief obtained by laser engraving is in particular that under laser irradiation the material passes directly into the gas phase with as far as possible no melting beforehand, since otherwise melt edges are formed around the depressions in the plate. Melt edges of this kind result in a considerable deterioration in the printed image and reduce the resolution of the printing plate and of the printed image.
For the economics of the process it is critical that the sensitivity of the recording material to laser radiation is as high as possible in order that the material can be laser-engraved extremely rapidly. In this context, however, it must be borne in mind that the laser-engravable layer is also required to have the performance properties that are important for relief printing plates, such as elasticity, hardness, roughness, ink acceptance, or low swellability in printing inks, for example. Optimizing the material in terms of laser engravability must certainly not result in any impairment in said performance properties.
Materials for producing relief printing plates by means of direct laser engraving are known in principle.
U.S. Pat. No. 3,549,733 discloses a polyoxymethylene or polychloral recording material for producing printing plates by means of laser engraving. Additionally, glass fibers or rutile can be used as fillers.
DE-A 196 25 749 discloses a seamless printing form (sleeve) for rotary flexographic printing, in which the elastomer layer is formed by a cold-curing silicone polymer or a silicone fluoropolymer, along with aluminum hydroxide as filler.
The sensitivity of the two systems to laser radiation, however, leaves something to be desired, with the consequence that imagewise engraving of the printing plate takes a long time.
EP-A 710 573 discloses a laser-engravable printing plate made from a polyurethane elastomer, nitrocellulose, and carbon black. The high levels of nonelastomeric nitrocellulose (from 25 to 45% by weight of the laser-sensitive layer), however, cause difficulties in the production of flexographic printing plates.
EP-A 640 043 and EP-A 640 044 disclose, respectively, single-layer and multilayer elastomeric laser-engravable elements for producing flexographic printing plates. The elements disclosed consist of xe2x80x9creinforcedxe2x80x9d elastomeric layers. Binders used are thermoplastic elastomers typical for flexographic printing plates, such as SBS, SIS or SEBS block copolymers, for example. The so-called reinforcement is achieved alternatively by means of fillers, photochemical crosslinking or thermochemical crosslinking, or combinations thereof. In addition, the layer may optionally include substances which absorb IR radiation. A preferred IR-absorbent material is carbon black, which at the same time also acts as filler. The engraving of elements with thermoplastic elastomers as binders using IR lasers, however, is often accompanied by the formation of melt edges, leading to defects in the printed image.
It is an object of the present invention to find an improved material for producing relief printing plates by means of laser engraving, which possesses an increased level of sensitivity to laser radiation and with which relief printing plates without melt edges can be produced.
We have found that this object is achieved by a laser-engravable recording material for producing relief printing plates, in particular for producing flexographic printing plates, comprising a dimensionally stable support and a recording layer comprising silicone rubbers and inorganic ferrous solids and/or carbon black as absorbers for laser radiation. We have also found a process for producing relief printing plates by engraving such recording materials using a laser, and relief printing plates having a printing relief comprising silicone rubbers and inorganic ferrous solids and/or carbon black as absorbers for laser radiation.
The recording material of the invention comprises a laser-engravable layer applied with or without an adhesion layer to a dimensionally stable support. Examples of suitable dimensionally stable supports are plates, films, and conical and cylindrical sleeves made from metals such as steel, aluminum, copper and nickel or from plastics such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polybutylene terephthalate, polyamide and polycarbonate, and, if desired, also woven and nonwoven materials, such as glass fiber fabrics, and also composite materials of glass fibers and plastics. Particularly suitable dimensionally stable supports are dimensionally stable support films, examples being polyester films, especially PET or PEN films.
The term xe2x80x9claser-engravablexe2x80x9d means that the layer possesses the property of absorbing laser radiation, especially the radiation of an IR laser, so that at those points where it is exposed to a laser beam of sufficient intensity it is removed, or at least detached. Preferably, the layer is vaporized without melting beforehand or is decomposed thermally or oxidatively, so that its decomposition products are removed from the layer in the form of hot gases, vapors, smoke, or small particles. However, the invention also embraces the subsequent mechanical removal of the residues of the irradiated layer by means, for example, of a jet of liquid or of gas, or else, for example, by suction.
The laser-engravable layer comprises at least one silicone rubber as binder. Silicone rubbers are formed by appropriate crosslinking of silicone polymers and are available commercially. Depending on the type of crosslinking, a distinction is made between heat-curing silicone rubbers (HV grades), cold-curing one-component silicone rubbers (RTV-1 grades), cold-curing two-component silicone rubbers (RTV-2 grades), and liquid silicone rubbers (LSR grades). A comprehensive description of silicone rubbers and the various curing techniques can be found, for example, in xe2x80x9cRubbers-5.1. Silicone Rubbersxe2x80x9d, Ullmann""s Encyclopedia of Industrial Chemistry, Sixth Edition, 1998, Electronic Release. The skilled worker will make an appropriate selection from the various types of silicone rubbers in accordance with the desired properties of the printing relief. In order to produce a laser-engravable recording element suitable for producing a flexographic printing plate, for example, the skilled worker will choose a relatively soft rubber, whereas for producing a relief or gravure printing plate he or she will choose harder grades. It is also possible to use blends of two or more silicone rubbers.
In addition, the properties of silicone rubbers can be influenced by means of additives such as fillers or plasticizers. Commercially available silicone rubbers contain in particular up to 50% by weight of pyrogenic or precipitated, unmodified or organically modified silica, quartz or alumina as fillers. Such additives of commercial silicone rubbers should be understood for the purposes of this invention as being included in the term silicone rubber.
It is also possible, furthermore, to use siloxane block copolymers having siloxane blocks and thermoplastic hard segments. Examples of such hard segment blocks are polycarbonate, polysulfone, and polyimide segments. Block copolymers of this kind have the properties of thermoplastic elastomers and for the purposes of this invention should be likewise understood as being embraced by the term silicone rubber.
The laser-engravable layer may, furthermore, include further polymeric binders different than silicone rubber. Additional binders of this kind can be used, for example, for controlled modification of the properties of the elastomeric layer. A prerequisite for the addition of further binders is that they are compatible with the silicone rubber. For example, other rubbers such as ethylene-propylene-diene rubbers are suitable for use as additional binders. The amount of additional binders is chosen by the skilled worker in accordance with the desired properties. Generally speaking, however, not more than 25% by weight, relative to the total amount of the binder used, preferably not more than 10% by weight, of such additional binders should be employed.
The recording layer of the invention further comprises an inorganic ferrous solid and/or carbon black as absorber for laser radiation. It is also possible to use mixtures of two or more absorbers for laser radiation. Suitable absorbers for laser radiation exhibit a high level of absorption in the region of the laser wavelength. Particularly suitable absorbers are those exibiting a high level of absorption in the near infrared and in the longer-wave VIS region of the electromagnetic spectrum. Absorbers of this kind are particularly suitable for absorbing radiation from Nd-YAG lasers (1064 nm) and from IR diode lasers, which typically have wavelengths of between 700 and 900 nm and between 1200 and 1600 nm.
Particularly suitable ferrous solids are intensely colored iron oxides. Iron oxides of this kind are available commercially and are conventionally used as color pigments or as pigments for magnetic recording. Examples of suitable absorbers for laser radiation are FeO, goethite xcex1-FeOOH, akaganeite xcex2-FeOOH, lepidokrokite xcex3-FeOOH, hematite xcex1-Fe2O3, maghemite xcex3-Fe2O3, magnetite Fe3O4 or berthollides. It is also possible to use doped iron oxides or mixed oxides of iron with other metals. Examples of mixed oxides are umbra Fe2O3xc3x97n MnO2 or FexAl(1xe2x88x92x)OOH, especially various spinel black pigments such as Cu(Cr,Fe)2O4, Co(Cr,Fe)2O4 or Cu(Cr,Fe,Mn)2O4. Examples of dopants are P, Si, Al, Mg, Zn and Cr, for example. Dopants of this kind are generally added in small amounts in the course of the synthesis of the oxides in order to control particle size and particle morphology. The iron oxides can also be in coated form. Such coatings can be applied, for example, in order to improve the dispersibility of the particles. These coatings can, for example, comprise inorganic compounds such as SiO2 and/or AlOOH. It is also possible, however, to apply organic coatings, examples being organic adhesion promoters such as aminopropyl(trimethoxy)silane. Particularly suitable absorbers for laser radiation are FeOOH, Fe2O3 and Fe3O4; Fe3O4 is especially preferred.
The size of the ferrous inorganic solids used, especially of the iron oxides, will be selected by the skilled worker in accordance with the desired properties of the recording material. Solids having an average particle size of more than 10 xcexcm, however, are generally unsuitable. Since iron oxides in particular are anisometric, this figure refers to the longest axis. The particle size is preferably less than 1 xcexcm. It is also possible to use what are known as transparent iron oxides, which have a particle size of less than 0.1 xcexcm and a specific surface area of up to 150 m2/g.
Further ferrous compounds suitable as absorbers for laser radiation are metal iron pigments. Particularly suitable are those pigments having a needle or rice-grain shape, with a length of between 0.1 and 1 xcexcm. Pigments of this kind are known for use as magnetic pigments for magnetic recording. In addition to iron, further dopants such as Al, Si, Mg, P, Co, Ni, Nd or Y can be present, or the metal iron pigments can be coated with them. Metal iron pigments are surface-anoxidized for protection against corrosion, and consist of a doped or undoped iron core and a doped or undoped iron oxide shell.
Suitable carbon blacks as absorbers for laser radiation are, in particular, finely divided grades of carbon black having a particle size of between 10 and 50 nm.
The amount of absorber added will be chosen by the skilled worker in accordance with the particular material being used and the desired properties of the recording material. In this context it should be borne in mind that the solids added as absorbers will affect not only the laser engravability but also, for example, the mechanical properties of the recording material, such as its hardness or other properties, e.g., the thermal conductivity. If, therefore, a relief or gravure printing plate harder than flexographic printing plates is to be produced, for example, the skilled worker will generally tend to select higher proportions of fillers than if the production of a flexographic printing plate were intended.
In general, however, more than 45% by weight of absorber, or mixtures of different absorbers, for laser radiation, relative to the sum of all of the constituents of the laser-engravable recording layer, is unsuitable. Preferably, the amount of the absorber for laser radiation is from 0.1 to 20% by weight, and with particular preference from 0.5 to 15% by weight.
In addition to the absorber for laser radiation the laser-engravable recording layer may also include further inorganic materials, especially oxides or oxide hydrates of metals, as fillers. These fillers serve, for example, to control the mechanical properties or the printing properties of the layer. Particular mention should be made here of SiO2, which is already a frequent constituent of commercially available silicone rubbers. Examples of others which can be used include TiO2, metal borides, metal carbides, metal nitrides, metal carbonitrides, metal oxides, and oxides having a bronze structure.
The laser-engravable recording layer can, furthermore, comprise auxiliaries and additives as well. Examples of such additives are colorants, plasticizers, dispersing auxiliaries, and adhesion promoters.
In general, the thickness of the laser-engravable recording layer is between 0.1 and 7 mm. The thickness will be suitably chosen by the skilled worker in accordance with the desired end use of the printing plate. The laser-engravable recording element may also comprise a plurality of laser-engravable recording layers, differing in composition, atop one another.
Optionally, the recording element of the invention may also include a thin top layer on the laser-engravable recording layer. By means of a top layer of this kind it is possible to modify parameters essential for the printing behavior and ink transfer, such as roughness, abrasiveness, surface tension, surface tack or solvent resistance at the surface, without affecting the printing plate properties typical to relief, such as hardness or elasticity, for example. In other words, surface properties and layer properties can be modified independently of one another in order to achieve an optimum print result. The top layer preferably also comprises a silicone rubber as polymeric binder, but may also include, conventionally, SIS or SBS block copolymers, for example. The top layer can comprise an absorber for laser radiation, although need not necessarily do so. The composition of the top layer is restricted only insofar as there must be no adverse effect on the laser engraving of the underlying laser-engravable layer and it must be possible to remove the top layer together with said laser-engravable layer. The top layer should be thin relative to the laser-engravable layer. As a general rule, the thickness of the top layer will not exceed 100 xcexcm; preferably, its thickness is situated between 5 and 80 xcexcm, with particular preference between 10 and 50 xcexcm.
The recording element of the invention may further optionally include a non-laser-engravable bottom layer situated between the support and the laser-engravable layer. Bottom layers of this kind make it possible to modify the mechanical properties of the relief printing plate without affecting the printing plate properties typical of the relief. As binder, the bottom layer may likewise comprise silicone rubbers or other polymers.
In addition, the laser-engravable recording element can optionally be protected against mechanical damage by means of a protective sheet of PET, for example, which is located on the respective topmost layer.
Production of the laser-engravable recording elements of the invention is oriented on the nature of the silicone rubber used. An essential factor for the quality of the recording material of the invention is that the absorber for the laser radiation and all other components are incorporated uniformly in the silicone rubber, so that a homogeneous recording material is formed. They can be produced, for example, by dissolving the starting polymer in an appropriate solvent such as toluene, for example, dispersing the absorber therein, with or without the addition of further auxiliaries, casting the resulting dispersion onto an appropriate support sheet, evaporating the solvent, and crosslinking the silicone polymer. This method is particularly advantageous when a cold-curing one-component system is being used. Furthermore, the recording materials of the invention can be produced, for example, by thoroughly mixing the starting components with one another in the absence of solvents in a dispersing apparatus, such as a compounder or extruder, for example, and shaping the mixture into a plate by means of compression molding, extrusion with a standard or circular die, injection molding, or any appropriate combination of techniques. Depending on the type of silicone rubber used, curing is carried out at room temperature or at elevated temperatures. The production process may also include aftertreatment steps such as calendering or grinding, for example. Steps of this kind are advantageously employed in order to obtain a recording material having an extremely smooth surface.
The laser-engravable recording materials of the invention are used as starting material for producing relief printing plates. The process involves first removing the cover film, if present. In the next step of the process, a printing relief is engraved into the recording material using a laser. Advantageously, the flanks of the image elements engraved drop vertically to start with and spread out only in the lower region of the image element. This provides good shoulder-shaping of the image dots but with low dot gain. Alternatively, image dot flanks with different configurations can be engraved.
Lasers particularly suitable for laser engraving are Nd-YAG lasers (1064 nm), IR diode lasers, which typically have wavelengths of between 700 and 900 nm and of between 1200 and 1600 nm, and CO2 lasers, having a wavelength of 10640 nm. It is also possible, however, to use lasers with shorter wavelengths, provided the laser is of sufficient intensity. For example, a frequency-doubled (532 nm) or frequency-tripled (355 nm) Nd-YAG laser can be used. Laser apparatus of this kind is available commercially. The image information to be engraved is transferred directly from the layout computer system to the laser apparatus. Laser operation can be either continuous or pulsed.
Laser engraving can be carried out advantageously in the presence of an oxygen-containing gas, especially air. The oxygen-containing gas can be blown over the recording element in the course or engraving. A comparatively gentle gas flow can be generated, for example, using a fan. It is also possible, however, to blow a stronger jet over the recording material with the aid of an appropriate nozzle. This embodiment has the advantage that solid constituents of the layer which have become detached can be effectively removed.
Optionally, the printing plate obtained can be cleaned further. A cleaning step of this kind removes constituents of the layer that have become detached but have not yet been completely removed from the surface of the plate. The printing plate can be cleaned, for example, using a brush. This cleaning process can be assisted by a suitable aqueous and/or organic solvent. A suitable solvent will be chosen by the skilled worker subject to the proviso that it does not dissolve or strongly swell the relief layer.
Alternatively, cleaning can be carried out, for example, with compressed air or by suction.
Although the recording materials of the invention are intended for laser engraving, the present invention also embraces mechanical engraving of the recording materials; that is, engraving by means, for example, of appropriate blades or other engraving tools.
With the process of the invention, relief printing plates are obtained whose printing relief has the same composition as the laser-engravable recording layer of the abovementioned recording element.
The examples which follow are intended to illustrate the invention, but do not restrict its scope.
Experimental details:
The engraving tests were carried out using a pulsed Nd-YAG laser (model: FOBA-LAS 94S, from Foba GmbH, Elektronik+Lasersysteme) having a wavelength of 1064 nm. A 2 mm mode diaphragm was used, and the velocity of the laser beam was 100 mm/s.
A pattern of 90 square engraving elements having an edge length of 2 mm each was engraved into the recording materials. The engraved elements were each separated from one another by thin webs of unengraved material (see FIG. 1). Both the laser output (by altering the lamp current) and the pulse frequency of the laser were increased in steps from one engraved element to the next. To engrave the entire pattern into the recording material took about 60 s. In each case the depth of 4 elements was evaluated, including the elements with lowest laser output and lowest pulse frequency and those with highest laser output and highest pulse frequency. The respective data are given in Table 1.